Global warming is a pressing, potentially disastrous problem for humanity. This has created a need for energy sources that do not emit greenhouse gasses, and that could supplant a substantial fraction of carbon-based energy supply on a relatively short time scale. Nuclear power that utilizes existing technology to provide the required amount of energy in a reasonable period of time, has been increasingly advocated as one strategy to combat global warming.
While renewable energy sources are advocated, their current state of development and intermittent nature limit the amount of energy they can supply. In terms of nuclear energy, since there is only a limited natural supply of fissionable isotopes from which usable energy can be derived, reactors that produce neutrons are required to convert fertile material to fissile material through a process known as breeding. However, current reactors are limited by the power density of neutrons they can supply. Oftentimes, because of these limitations, nuclear fuel breeding processes are extremely inefficient, and as such, require vast amounts fertile material to provide sufficient amounts of fissile material. Such low yields create further challenges, such as nuclear waste management and weapons proliferation control. In addition, because of low yielding reactors, many fuel supplying reactors are needed to supply fission reactors, leading again to many of the aforementioned challenges. It has been suggested that nuclear fusion reactors could supply the large number of neutrons to breed nuclear fuel for fission reactors. However, currently, nuclear fusion technology is insufficient for practically supplying enough nuclear fuel to fission reactors for usable energy production. In theory, however, nuclear fusion can potentially offer a practical approach to nuclear fuel supply.
Nuclear fusion is a source of neutrons and energy derived from nuclear combinations of light elements into heavier elements resulting in a release of energy. In fusion, two light nuclei (such as deuterium and tritium) combine into one new nucleus (such as helium) and release enormous energy and another particle (such as a neutron in the case of the fusion of deuterium and tritium) in the process. Nuclear fusion is more neutron-rich energy source than fission. While fusion is a spectacularly successful energy source for the sun and the stars, the practicalities of harnessing fusion on Earth are technically challenging, given that to sustain fusion, a plasma (a gas consisting of charged ions and electrons), or an ionized gas, has to be confined and heated to millions of degrees Celsius in a fusion reactor for a sufficient period of time to enable the fusion reaction to occur. The science behind fusion is well advanced, rooted in more than 100 years of nuclear physics and electromagnetic and kinetic theory, yet current engineering constraints make the practical use of nuclear fusion as a direct energy source very challenging. One approach to fusion reactors uses a powerful magnetic field to confine plasma, thereby releasing fusion energy in a controlled manner. To date, the most successful approach for achieving controlled fusion is in a donut-shape or toroidal-shape magnetic configuration called a tokamak. While a tokamak can, in principle, be used as a source of the fast neutrons needed breeding fissile materials, the current art of fusion reactors limits tokamaks to power densities that are far too low (by factors of 5 or more) for this purpose.
With current tokamak technology, the confinement of plasma to produce nuclear fusion reactions can be accomplished with a magnetic field (i.e., a magnetic bottle) created inside a vacuum chamber of a fusion reactor. Since the plasma is ionized, plasma particles tend to gyrate in small orbits around magnetic field lines, i.e., they essentially stick to the magnetic field lines, while flowing quite freely along the field lines. This can be used to “suspend” bulk plasma in the vacuum chamber by using a properly designed magnetic field configuration, which is sometimes called a magnetic bottle. The plasma can be magnetically contained within the chamber by creating a set of nested toroidal magnetic surfaces by driving an electric current in the plasma, and by the placement of current-carrying coils or conductors adjacent to the plasma. Since magnetic field lines on these magnetic surfaces do not touch any material objects such as walls of the vacuum chamber, the very hot plasma can ideally remain suspended in the magnetic bottle, i.e., in the volume containing closed magnetic surfaces, for a long time, without the particles coming into contact with the walls. However, in reality, particles and energy very slowly escape magnetic confinement in a direction perpendicular to the magnetic surfaces as a result of particle collisions with one another or turbulence in the plasma. Decreasing this slow plasma loss, so that the particles and energy of the plasma are better confined, has been a fundamental focus of plasma confinement research.
The boundary of the magnetic bottle containing closed magnetic surfaces, i.e., the “core plasma”, is defined by either material objects called limiters (e.g., 610 with reference to FIG. 6), or by a toroidal magnetic surface called a separatrix (e.g., 630 with reference to FIG. 6), outside of which the magnetic field lines are “open”, i.e., they terminate on material objects called divertor targets (e.g., 620 with reference to FIG. 6). The particles and energy slowly escaping the core plasma mainly fall on small areas of either limiter or divertor targets and generate impurities. Since limiters are right at the plasma boundary, while divertor targets can be placed farther away, core plasma can be better isolated from such impurities by using divertors. Since the invention of divertors, the preferred mode of plasma operation has been to have a separatrix and a divertor, since such operation has been found to enable a mode of operation called the H-mode, where the plasma particles and energy in the core are better confined.
Since particles flow very fast along magnetic lines but very slow across them, any particles and energy that escape across the separatrix reach divertor targets quickly along open field lines before moving much across them. This creates a necessarily narrow “scrape-off layer” with a high “scrape off flux” of particles and energy that falls on narrow areas of the divertor plates. The maximum “scrape off flux” that a divertor can handle limits the highest power density that can be sustained in a magnetic bottle.
High “scrape off flux” creates a multitude of challenges. In addition to heat and particle fluxes, the divertor plates also have to withstand large fluxes of neutrons created in fusion. These neutrons cause a degradation of many important material properties, making it extremely difficult for a divertor plate to handle both the high heat fluxes and neutron fluxes without having to be replaced frequently. Periodically replacing the damaged components is very time consuming and requires the fusion reaction to be shut off. Further, trying to reduce the “scrape off flux” by injecting impurities to radiate energy before it reaches divertor plates is not workable because the density of power coming out of the plasma becomes so high that it seriously degrades the plasma confinement, which results in a serious reduction of the fusion reaction rate in the core plasma.
To lower neutron and heat fluxes on a divertor and thus mitigate the damage to a divertor component, a reactor could simply be made larger to decrease the density of power within a device. However, this approach significantly increases the reactor cost, and hence the cost of any energy produced with it, to levels that are economically non-competitive with other methods for the generation of power or neutrons.
A high level of “scrape off flux” is a critical roadblock for many fusion applications, including nuclear fuel breeding. For example, for fusion reactors of sizes that can make them economically competitive with other methods of energy production, the high “scrape off flux” is intolerable for divertor designs based on current art. One way of handling challenges presented by high scrape off flux and enabling compact high-power density fusion neutron sources is described in U.S. patent application Ser. No. 12/197,736 to Kotschenreuther, et al, filed Aug. 25, 2008, fully incorporated herein by reference and made a part hereof.
Therefore, there remains a need for improved nuclear fusion reactors to provide sufficient flux and fluence (i.e., time-integrated flux) of fast neutrons with sufficient energy to breed nuclear fuel from fertile material so as to effectively overcome challenges in the current art, some of which are mentioned above.